1. Field of the Invention
The present invention relates to a method and apparatus for stabilizing output beam parameters of a gas discharge laser. More particularly, the present invention relates to maintaining an optimal gas mixture composition over long, continuous operating or static periods using very small gas injections.
2. Discussion of the Related Art
Pulsed gas discharge lasers such as excimer and molecular lasers emitting in the deep ultraviolet (DUV) or vacuum ultraviolet (VUV) have become very important for industrial applications such as photolithography. Such lasers generally include a discharge chamber containing two or more gases such as a halogen and one or two rare gases. KrF (248 nm), ArF (193 nm), XeF (350 nm), KrCl (222 nm), XeCl (308 nm), and F2 (157 nm) lasers are examples.
The efficiencies of excitation of the gas mixtures and various parameters of the output beams of these lasers vary sensitively with the compositions of their gas mixtures. An optimal gas mixture composition for a KrF laser has preferred gas mixture component ratios around 0.1% F2 /1% Kr/98,9% Ne (see U.S. Pat. No. 4,393,505, which is assigned to the same assignee as the present application and is hereby incorporated by reference into the present application). A F2 laser may have a gas component ratio around 0.1% F2/99.9% Ne (see U.S. patent application Ser. No. 09/317,526, which is assigned to the same assignee as the present application and is hereby incorporated by reference into the present application). Small amounts of Xe may be added to rare gas halide gas mixtures, as well (see R. S. Taylor and K. E. Leopold, Transmission Properties of Spark Preionization Radiation in Rare-Gas Hailde Laser Gas Mixes, IEEE Journal of Quantum Electronics, pp. 2195-2207, vol. 31, no. 12 (December. 1995). Any deviation from the optimum gas compositions of these or other excimer or molecular lasers would typically result in instabilities or reductions from optimal of one or more output beam parameters such as beam energy, energy stability, temporal pulse width, temporal coherence, spatial coherence, discharge width, bandwidth, and long and short axial beam profiles and divergences.
Especially important in this regard is the concentration (or partial pressure) of the halogen, e.g., F2, in the gas mixture. The depletion of the rare gases, e.g., Kr and Ne for a KrF laser, is low in comparison to that for the F2. FIG. 1 shows laser output efficiency versus F2 concentration for a KrF laser, showing a decreasing output efficiency away from a central maximum. FIG. 2 shows how the temporal pulse width (pulse length or duration) of KrF laser pulses decrease with increasing F2 concentration. FIGS. 3-4 show the dependence of output energy on driving voltage (i.e., of the discharge circuit) for various F2 concentrations of a F2 laser. It is observed from FIGS. 3-4 that for any given driving voltage, the pulse energy decreases with decreasing F2 concentration. In FIG. 3, for example, at 1.9 kV, the pulse energies are around 13 mJ, 11 mJ and 10 mJ for F2 partial pressures of 3.46 mbar, 3.16 mbar and 2.86 mbar, respectively. The legend in FIG. 3 indicates the partial pressures of two premixes, i.e., premix A and premix B, that are filled into the discharge chamber of a KrF laser. Premix A comprised substantially 1% F2 and 99% Ne, and premix B comprised substantially 1% Kr and 99% Ne. Therefore, for the graph indicated by triangular data points, a partial pressure of 346 mbar for premix A indicates that the gas mixture had substantially 3.46 mbar of F2 and a partial pressure of 3200 mbar for premix B indicates that the gas mixture had substantially 32 mbar of Kr, the remainder of the gas mixture being the buffer gas Ne. FIG. 5 shows a steadily increasing bandwidth of a KrF laser with increasing F2 concentration.
In industrial applications, it is advantageous to have an excimer or molecular fluorine laser capable of operating continuously for long periods of time, i.e., having minimal downtime. It is desired to have an excimer or molecular laser capable of running non-stop year round, or at least having a minimal number and duration of down time periods for scheduled maintenance, while maintaining constant output beam parameters. Uptimes of, e.g., greater than 98% require precise control and stabilization of output beam parameters, which in turn require precise control of the composition of the gas mixture.
Unfortunately, gas contamination occurs during operation of excimer and molecular fluorine lasers due to the aggressive nature of the fluorine or chlorine in the gas mixture. The halogen gas is highly reactive and its concentration in the gas mixture decreases as it reacts, leaving traces of contaminants. The halogen gas reacts with materials of the discharge chamber or tube as well as with other gases in the mixture. Moreover, the reactions take place and the gas mixture degrades whether the laser is operating (discharging) or not. The passive or static gas (i.e., when the laser is not discharging, or operating) lifetime is about one week for a typical KrF-laser.
During operation of a KrF-excimer laser, such contaminants as HF, CF4, COF2, SiF4 have been observed to increase in concentration rapidly (see G. M. Jurisch et al., Gas Contaminant Effects in Discharge-Excited KrF Lasers, Applied Optics, Vol. 31, No. 12, pp. 1975-1981 (Apr. 20, 1992)) For a static KrF laser gas mixture, i.e., with no discharge running, increases in the concentrations of HF, O2, CO2 and SiF4 have been observed (see Jurisch et al., above).
One way to effectively reduce this gas degradation is by reducing or eliminating contamination sources within the laser discharge chamber. With this in mind, an all metal, ceramic laser tube has been disclosed (see D. Basting et al., Laserrohr fxc3xcr halogenhaltige Gasentladungslaserxe2x80x9d G 295 20 280.1, Jan. 25, 1995/Apr. 18, 1996 (disclosing the Lambda Physik Novatube, and hereby incorporated by reference into the present application)). FIG. 6 qualitatively illustrates how using a tube comprising materials that are more resistant to halogen erosion (plot B) can slow the reduction of F2 concentration in the gas mixture compared to using a tube which is not resistant to halogen erosion (plot A). The F2 concentration is shown in plot A to decrease to about 60% of its initial value after about 70 million pulses, whereas the F2 concentration is shown in plot B to decrease only to about 80% of its initial value after the same number of pulses. Gas purification systems, such as cryogenic gas filters (see U.S. Pat. No. 4,534,034) or electrostatic particle filters (see U.S. Pat. No. 5,586,134) are also being used to extend KrF laser gas lifetimes to 100 million shots before a new fill is advisable.
It is not easy to directly measure the halogen concentration within the laser tube for making rapid online adjustments (see U.S. Pat. No. 5,149,659 (disclosing monitoring chemical reactions in the gas mixture)). Therefore, it is recognized in the present invention that an advantageous method applicable to industrial laser systems includes using a known relationship between F2 concentration and a laser parameter, such as one of the F2 concentration dependent output beam parameters mentioned above. In such a method, precise values of the parameter would be directly measured, and the F2 concentration would be calculated from those values. In this way, the F2 concentration may be indirectly monitored.
Methods have been disclosed for indirectly monitoring halogen depletion in a narrow band excimer laser by monitoring beam profile (see U.S. Pat. No. 5,642,374) and spectral (band) width (see U.S. Pat. No. 5,450,436). Neither of these methods is particularly reliable, however, since beam profile and bandwidth are each influenced by various other operation conditions such as repetition rate, tuning accuracy, thermal conditions and aging of the laser tube. That is, the same bandwidth can be generated by different gas compositions depending on these other operating conditions.
It is known to compensate the degradation in laser efficiency due to halogen depletion by steadily increasing the driving voltage of the discharge circuit to maintain the output beam at constant energy. To illustrate this, FIG. 7 shows how at constant driving voltage, the energy of output laser pulses decreases with pulse count. FIG. 8 then shows how the driving voltage may be steadily increased to compensate the halogen depletion and thereby produce output pulses of constant energy.
One drawback of this approach is that output beam parameters other than energy such as those discussed above with respect to FIGS. 1-5 affected by the gas mixture degradation will not be correspondingly corrected by steadily increasing the driving voltage. FIGS. 9-11 illustrate this point showing the driving voltage dependencies, respectively, of the long and short axis beam profiles, short axis beam divergence and energy stability sigma. Moreover, at some point the halogen becomes so depleted that the driving voltage reaches its maximum value and the pulse energy cannot be maintained without refreshing the gas mixture.
It is desired to have a method of stabilizing all of the output parameters affected by halogen depletion and not just the energy of output pulses. It is recognized in the present invention that this is most advantageously achieved by adjusting the halogen and rare gas concentrations themselves.
There are techniques available for replenishing a gas mixture by injecting additional rare and halogen gases into the discharge chamber and readjusting the gas pressure (see U.S. Pat. No. 5,396,514). A more complex system monitors gas mixture degradation and readjusts the gas mixture using selective replenishment algorithms for each gas of the gas mixture (see U.S. Pat. No. 5,440,578). A third technique uses an expert system including a database of information and graphs corresponding to different gas mixtures and laser operating conditions (see U.S. patent application Ser. No. 09/167,653, assigned to the same assignee as the present application and hereby incorporated by reference into the present application). A data set of driving voltage versus output pulse energy, e.g., is measured and compared to a stored xe2x80x9cmasterxe2x80x9d data set corresponding to an optimal gas composition such as may be present in the discharge chamber after a new fill. From a comparison of values of the data sets and/or the slopes of graphs generated from the data sets, a present gas mixture status and appropriate gas replenishment procedures, if any, may be determined and undertaken to reoptimize the gas mixture. Some gas replenishment procedures are described in U.S. Pat. No. 4,977,573, which is assigned to the same assignee as the present application and is hereby incorporated by reference into the present application.
All of these techniques generally produce undesirable disturbances in laser operation conditions when the gas is replenished. For example, strong pronounced jumps of the driving voltage are produced as a result of halogen injections (HI) as illustrated in FIG. 12. The result is a strong distortion of meaningful output beam parameters such as the pulse-to-pulse stability. For this reason, the laser is typically shut down and restarted for gas replenishment, remarkably reducing laser uptime (see U.S. Pat. No. 5,450,436).
It is an object of the invention to provide an excimer or molecular laser system, wherein the gas mixture status may be precisely and periodically determined and smoothly adjusted.
It is a further object of the invention to provide a technique which automatically compensates gas mixture degradation without disturbing laser operation conditions when the gas is replenished.
It is another object of the invention to provide an excimer or molecular laser system capable of running continuously while maintaining constant output beam parameters.
A gas discharge laser system is provided to meet the above objects which has a discharge chamber containing a first gas mixture including a constituent gas, a pair of electrodes connected to a power supply circuit including a driving voltage for energizing the first gas mixture, and a resonator surrounding the discharge chamber for generating a laser beam. A gas supply unit is connected to the discharge chamber for replenishing the first gas mixture including the constituent gas. The gas supply unit includes a gas inlet port having a valve for permitting a second gas mixture to inject into the discharge chamber to mix with the first gas mixture therein. A processor monitors a parameter indicative of the partial pressure of the constituent gas and controls the valve at successive predetermined intervals to compensate a degradation of the constituent gas in the first gas mixture. The pressure of the first gas mixture is increased by an amount between 0.01 and 10 mbar, and preferably between 0.1 and 1 or 2 mbar, as a result of each successive injection. The second gas mixture preferably comprises 1% of the first constituent gas, preferably a halogen containing species, and 99% buffer gas, so that the partial pressure of the constituent gas in the discharge chamber increases by between 0.0001 and 0.1 mbar, and preferably between 0.001 and 0.01 or 0.02 mbar, as a result of each successive injection. The processor monitors the parameter indicative of the partial pressure of the constituent gas wherein the parameter varies with a known correspondence to the partial pressure of the constituent gas. The small gas injections each produce only small variations in partial pressure of the constituent gas in the first gas mixture, and thus discontinuities in laser output beam parameters are reduced or altogether avoided.
A method is also provided for stabilizing laser output by maintaining a constituent gas of the laser gas mixture at a predetermined partial pressure within the discharge chamber of the gas discharge laser using a gas supply unit and a processor. The method begins with providing a laser gas mixture including a constituent gas at an initial partial pressure which is subject to depletion within the discharge chamber. Next, a parameter which varies with a known correspondence to the partial pressure of the constituent gas is monitored. A value of the parameter corresponding to a selected partial pressure or partial pressure reduction of the constituent gas is selected and injections of the constituent gas, or a gas mixture including the constituent gas, are performed when predetermined values or changes in value of that parameter are measured. The gas pressure within the discharge chamber is increased by between 0.01 and 10 mbar, or preferably between 0.1 and 1 or 2 mbar, with each injection, when a gas mixture including around 1% of the constituent gas is injected. The constituent gas partial pressure increases by between 0.0001 and 0.1 mbar, or preferably between 0.001 and 0.01 and 0.02 mbar, with each injection. The injections are performed at intervals sufficient to maintain or return the constituent gas substantially at or to the initial partial pressure within the laser gas mixture.
The constituent gas is typically a halogen containing species such as fluorine or hydrogen chloride. The constituent gas may be an active rare gas or gas additive. The monitored parameter is preferably selected from the group including time, pulse count, total accumulated energy input to the discharge, driving voltage for maintaining a constant laser beam output energy, pulse shape, pulse duration, pulse stability, beam profile, bandwidth of the laser beam, energy stability, moving average energy close, temporal pulse width, temporal coherence, spatial coherence, discharge width, and long and short axial beam profiles and divergences, or a combination thereof. Each of these parameters varies with a known correspondence to the partial pressure of the halogen, and that halogen partial pressure is then precisely controlled using the small gas injections to provide stable output beam parameters.
In the present invention, the gas supply unit preferably includes a small gas reservoir for storing the constituent gas or second gas mixture prior to being injected into the discharge chamber (see U.S. Pat. No. 5,396,514, which is assigned to the same assignee as the present application and is hereby incorporated by reference into the present application, for a general description of how such a gas reservoir may be used). The reservoir may be the volume of the valve assembly or an additional accumulator. The accumulator is advantageous for controlling the amount of the gas to be injected. The pressure and volume of the second gas mixture are selected so that the pressure in the discharge chamber will increase by a predetermined amount less than 10 mbar, and preferably between 0.1 and 2 mbar, with each injection. These preferred second gas mixture partial pressures may be varied depending on the percentage concentration of the halogen containing species in the second gas mixture.
Injections may be continuously performed during operation of the laser in selected amounts and at selected small intervals. Alternatively, a series of injections may be performed at small intervals followed by periods wherein no injections are performed. The series of injections followed by the latent period would then be repeated at predetermined larger intervals.